1. Field of the Invention
The present invention relates to a pneumatic tire for two-wheeled motor vehicles capable of improving the tire strength without harming the steering stability.
2. Description of the Prior Art
A variety of pneumatic tires for two-wheeled motor vehicles provided with belt layers outside of carcasses of radial structure have been suggested in the past. In these years, particularly those employing so-called jointless belt plies as belt layers in which steel cords are spirally wound in a substantially parallel manner with respect to a tire circumferential direction have been suggested. Since such belt layers exhibit strong constraining force in circumferential directions and further include no joints, they have the advantage of exhibiting superior uniformity. It is another advantage that lack in traction at the time of performing turning movements owing to lack in tread rigidity can be solved, which might cause problems particularly in large-sized vehicles, by the use of steel cords.
However, with such a jointless belt layer, effects of reinforcing the tread portion will become less when compared to a belt layer comprised of conventional cut end plies in which belt cords are mutually crossed and overlapped. Therefore, for clearing the strength test as provided by JIS K6366, it will be necessary to comprise the carcass of a plurality of carcass plies and/or to increase the cord implantation number for the belt layer.
However, the above-mentioned reinforcing methods are disadvantageous in that increases in tire weights are caused in addition to degraded performance of absorbing disturbances and riding comfort. The inventors of the present invention have thus suggested providing a reinforcing layer of thin organic fiber cords outside of the belt layer. However, a simple provision of a reinforcing layer will lead to a new drawback that the rigidity of the tread portion is increased to thus worsen the steering stability at the time of turning.
The present invention has been made in view of the above facts, and it is an object thereof to improve the tire strength in a pneumatic tire for two-wheeled motor vehicles including a jointless belt layer in which steel cords are spirally wound while securing its advantages and further restricting increases in tire weight, degradations in disturbance absorbing performance and riding comfort, and also degradations in steering stability at the time of performing turning movements.
The invention as recited in claim 1 of the present invention is a pneumatic tire for two-wheeled motor vehicles in which a tread width comprises a tire maximum width and in which a tread surface warps in an arc-like manner within a tire meridian section, the tire comprising:
a carcass comprised of a single carcass ply including organic fiber cords that extend between a pair of bead portions in a toroidal manner,
a belt layer disposed outside of the carcass in a radial direction and comprised of at least one belt ply formed by spirally winding a tape-like member of small width obtained by coating one or a plurality of steel cords with topping rubber so that the steel cords are substantially parallel to a tire circumferential direction, and
a tread reinforcing layer disposed outside of the belt layer in the tire radial direction and comprised of a single reinforcing ply in which organic fiber cords are aligned to be substantially parallel to the tire circumferential direction,
wherein the reinforcing ply has a width in a tire axial direction of not less than 30 mm and not more than 90% of a tread grounding width.
In the present specification, the term “tread grounding width” denotes a maximum grounding width in the tire axial direction when standard load is applied to the pneumatic tire for two-wheeled motor vehicles in a standard internal pressure condition and is grounded on a flat surface at a camber angle of 0 degrees. The term “standard condition” denotes a condition in which the tire is assembled to a standard rim and is in a no-load condition filled with standard internal pressure.
The term “standard rim” denotes a rim with standards being defined for each tire within standardizing systems including standards, on which the tires are based, such concretely being a standard rim according to JATMA, a “design rim” according to TRA and a “measuring rim” according to ETRTO. The term “standard internal pressure” denotes an air pressure that is defined by the standards for each of the tires, and it denotes a maximum air pressure according to JATMA, a maximum value as recited in the table of “tire load limits at various cold inflation pressures” according to TRA, and “inflation pressure” according to ETRTO. The term “standard load” denotes a load that is defined by the standards for each of the tires, and it denotes a maximum load performance according to JATMA, a maximum value as recited in the table of “tire load limits at various cold inflation pressures” according to TRA, and “load capacity” according to ETRTO.
In the present specification, the term “substantially parallel to the tire circumferential direction” denotes a condition in which an angle formed with respect to the tire circumferential direction is not more than 5 degrees.
The pneumatic tire for two-wheeled motor vehicles according to the present invention employs a jointless ply in which steel cords are spirally wound as the belt layer. Accordingly, lack in traction during turning movements can be solved particularly in large-sized vehicles while securing superior uniformity. Further, by the provision of a reinforcing ply made of organic fiber cords of specific cord arrangements and width outside of the belt layer, it is possible to improve the tire strength while respectively restricting increases in tire weight, degradations in disturbance absorbing performance and riding comfort and also degradations in steering stability when performing turning movements.